1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device having a nitride semiconductor layer.
2. Description of the Related Art
In recent years, the use of nitride semiconductor devices as light sources for large capacity optical disc has been expected and the development has become active. Moreover, conventionally, there are known nitride semiconductor laser devices in which a nitride semiconductor layer is formed on a GaN substrate as a nitride semiconductor substrate (see, for example, Japanese Unexamined Application No. 2000-140052).
Japanese Unexamined Application No. 2000-140052 discloses a structure of a nitride semiconductor laser device formed by sequentially growing an n-type layer, an n-type cladding layer made of an AlGaN layer, a luminous layer including an InGaN active layer with a multiple quantum well (MQW) structure, and a p-type cladding layer made of an AlGaN layer on an n-type GaN substrate. In the nitride semiconductor laser device described in Japanese Unexamined Application No. 2000-140052, the InGaN active layer is sandwiched by AlGaN cladding layers that occupy most of the thickness of the light-emitting device structure formed on the GaN substrate.
However, the conventional nitride semiconductor laser device described in Japanese Unexamined Application No. 2000-140052 has the following drawback. In the process of forming a layered structure of the nitride semiconductor laser device on the GaN substrate by crystal growth, a warp and a crack are generated in the GaN substrate by a lattice constant difference between the GaN substrate and each of the AlGaN cladding layers that occupy most of the thickness of the light-emitting device structure. This disadvantageously causes a reduction in yield of the nitride semiconductor laser device.
Further, conventionally, a structure of a semiconductor light-emitting device is known which suppresses occurrence of a warp and occurrence of a crack in the substrate by using an AlGaN substrate having substantially the same Al composition ratio as that of an AlGaN cladding layer in order to reduce a lattice constant difference between the AlGaN cladding layer and the substrate. Unlike the one disclosed in Japanese Unexamined Application No. 2000-140052, the structure of this conventional semiconductor light-emitting device makes it possible to suppress a reduction in yield caused by the warp and the crack in the substrate.
In the aforementioned conventional semiconductor light-emitting device in which the Al composition ratio of the AlGaN cladding layer is substantially the same as that of the AlGaN substrate, it is possible to suppress the reduction in yield caused by the warp and the crack in the substrate. A decrease, however, in absorption power of Al, Ga, and N which are constituent elements of the nitride semiconductor layer growing on the AlGaN substrate leads to a drawback. This tends to create pit and dislocation in the light-emitting device grown on the AlGaN substrate, causing a problem of a deteriorated device characteristic of the semiconductor device.
Moreover, in the nitride semiconductor laser device described in Japanese Unexamined Application No. 2000-140052 has the following drawback. The InGaN active layer is sandwiched by AlGaN cladding layers that occupy most of the thickness of the light-emitting device structure formed on the substrate. As a result, strain occurs in the active layer due to the lattice constant difference between the AlGaN cladding layer and the InGaN substrate. This disadvantageously causes a reduction in luminous efficiency due to the strain in the active layer (luminous layer). As a result, this also causes a problem of a deteriorated device characteristic.